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Desert sand from a Middle East country was melted into calcium magnesium aluminosilicate (CMAS) glass. Its chemical composition was analyzed to be 25.2CaO-2.6MgO-8.2Al2O3-59.8SiO2-1.6Fe2O3-1.5K2O weight % using inductively coupled plasma-atomic emission spectrometry. The CMAS glass powder was hot pressed into billets. Slow-crack-growth (SCG) and indentation deformation/fracture of the CMAS glass was investigated. The SCG susceptibility parameter (n) was found to be 25 ± 3 which is within a range of n = 15–35 that has been observed in many silicate glasses and glass ceramics. A similarity in indentation hardness and toughness was found between the CMAS glass and the low-silica content (50–70%) glasses. However, an exception was that significant lateral cracking was typified in the CMAS glass, as quantified via stress analysis in the vicinity of an indent.

The response of a silicon carbide (SiC) fibrous ceramic composite to foreign object damage (FOD) was determined at ambient temperature and velocities ranging from 40 to 150 m/s. Target specimens were impacted, at a normal incidence angle and in a partially supported configuration, using 1.59 mm diameter hardened steel ball projectiles. Qualitative analysis of the damage morphologies of targets and projectiles was made via scanning electron microscopy (SEM). In addition, the extent of impact damage was characterized by determining the post-impact strength of each target specimen as a function of impact velocity. Relative to the as-received (As-R) strength, the fibrous composite showed limited strength degradation due to impact with the maximum reduction of 17% occurring at 150 m/s. A quasi-static analysis of the impact force prediction was also made based on the principle of energy conservation and the results were verified via experimental data.

Erosion behavior of a large number of gas-turbine grade ceramic matrix composites (CMCs) was assessed using fine to medium grain garnet erodents at velocities of 200 and 300 m/s at ambient temperature. The CMCs used in the current work were comprised of nine different SiC/SiCs, one SiC/C, one C/SiC, one SiC/MAS, and one oxide/oxide. Erosion damage was quantified with respect to erosion rate and the damage morphology was assessed via SEM and optical microscopy in conjunction with 3-D image mapping. The CMCs response to erosion appeared to be very complicated due to their architectural complexity, multiple material constituents, and presence of pores. Effects of architecture, material constituents, density, matrix hardness, and elastic modulus of the CMCs were taken into account and correlated to overall erosion behavior. The erosion of monolithic ceramics such as silicon carbide and silicon nitrides was also examined to gain a better understanding of the governing damage mechanisms for the CMC material systems used in this work.

Foreign object-damage (FOD) phenomena of an N720/alumina oxide/oxide ceramic matrix composite (CMC), impacted by 1.59-mm spherical chrome steel projectiles up to Mach 1, were assessed at ambient temperature at a normal incidence angle in both partial and full supports. The impact damage was in the form of craters, matrix/fiber tow breakage, compaction of the material, delamination and cone cracks, and their occurrence and degree depended on both impact velocity and type of target supports. The partial support resulted in significant damage with increasing impact velocity, accompanying substantial strength degradation. The presence of tensile stress and presumably stress wave interaction at the backside of a target could have been responsible for greater impact damage in partial support. Although the CMC targets impacted at 340 m/s were on the verge of being penetrated, the targets still survived catastrophic failure retaining about 68% of the as-received strength, indicative of relatively superior FOD resistance as compared to monolithic ceramic counterparts. A quasi-static analysis of impact force prediction was made based on the energy balance principle and was validated indirectly using the experimental data on frontal impact damage size.

Assessments of foreign object damage (FOD) of a commercial, gas-turbine grade, in situ toughened silicon nitride ceramic (AS800) were made using different projectile materials at ambient temperature. AS800 flexure target specimens rigidly supported were impacted at their centers in a velocity range from 100 to 450 m/s by spherical projectiles with a diameter of 1.59 mm. Three different projectile materials were used including hardened steel, annealed steel, and brass. Post-impact strength of each target specimen impacted was determined as a function of impact velocity to appraise the severity of local impact damage. For a given impact velocity, the extent of FOD was greatest for hardened steel projectiles, least for brass projectiles, and intermediate for annealed steel projectiles. The key material parameter affecting FOD the most was identified as the hardness (or yield stress) of projectile materials. Prediction of impact force as a function of impact velocity for each projectile material was made based on a quasi-static plastic model incorporated with the average ‘contact yield pressure’ determined from static indentation testing.

Foreign object damage (FOD) phenomenon of an N720/alumina oxide/oxide ceramic matrix composite (CMC) was determined previously using 1.59 mm-diameter hardened steel ball projectiles using impact velocities ranging from 150 to 350 m/s at a normal incidence angle. Target specimens were impacted under tensile preloading with three different levels of load factors of 0, 30, and 50%. Difference in impact damage between no-preload and pre-load was significant particularly at 350 m/s with the highest load factor of 50%. A fracture mechanics approach was used to predict post-impact strength as a function of load factor and was assessed its applicability using the experimental data.

The life prediction analysis based on an exponential crack-velocity formulation was made and examined using a variety of experimental data on advanced structural ceramics in constant stress (‘static fatigue’ or ‘stress rupture’) testing at ambient and elevated temperatures. The data fit to the relation between ln (time to failure) versus applied stress was be very reasonable for most of the materials studied, resulting in a similar degree of accuracy as compared with the power-law crack-velocity formulation. The major limitation in the exponential crack-velocity formulation, however, was that the inert strength of a material must be known priori to evaluate the important slow-crack-growth (SCG) parameter n, a drawback as compared with the conventional power-law crack-velocity formulation.

The life prediction analysis based on an exponential crack-velocity formulation was made and examined using a variety of experimental data on glass and advanced structural ceramics in constant stress-rate (‘dynamic fatigue’) loading at ambient and elevated temperatures. The data fit to the relation of strength versus ln (stress rate) was very reasonable for most of the materials. The major limitation in the exponential crack-velocity formulation, however, was that the inert strength of a material must be known a priori to evaluate the important SCG parameter, n, a significant drawback as compared with the conventional power-law crack-velocity formulation.

Ceramic thermal barrier coatings (TBCs) will play an increasingly important role in future gas turbine engines because of their ability to effectively protect the engine components and further raise engine temperatures. Durability of the coating systems remains a critical issue with the ever-increasing temperature requirements. Thermal conductivity increase and coating degradation due to sintering and phase changes are known to be detrimental to coating performance. There is a need to characterize the coating thermal fatigue behavior and temperature limit, in order to potentially take full advantage of the current coating capability. In this study, a laser thermal fatigue test technique has been used to study the delamination crack propagation of thermal barrier coatings under simulated engine heat flux heating and thermal cyclic loading. Thermal conductivity and cyclic fatigue behaviors of plasma-sprayed ZrO2–8 wt.% Y2O3 thermal barrier coatings were evaluated under high temperature, large thermal gradient and thermal cycling conditions. The coating degradation and failure processes were assessed by real-time monitoring of the coating thermal conductivity during the testing. The test results showed that the initial average crack propagation rates of ZrO2–8 wt.% Y2O3 coatings were in the range of 3–8 μm/cycle. The crack propagation rates increased to 30–40 μm/cycle at later stages of testing, and the critical spalling crack size ranged from 3 to 5 mm for the coatings. The accelerated crack growth is attributed to the increased driving force for crack propagation under the laser heat flux cyclic test conditions.

Foreign object damage (FOD) behavior of two commercial gas-turbine grade silicon nitrides, AS800 and SN282, was determined at ambient temperature through strength testing of flexure test specimens impacted by steel-ball projectiles with a diameter of 1.59 mm in a velocity range from 220 to 440 m/s. AS800 silicon nitride exhibited a greater FOD resistance than SN282, primarily due to its greater value of fracture toughness (K(sub IC)). Additionally, the FOD response of an equiaxed, fine-grained silicon nitride (NC132) was also investigated to provide further insight. The NC132 silicon nitride exhibited the lowest fracture toughness of the three materials tested, providing further evidence that K(sub IC) is a key material parameter affecting FOD resistance. The observed damage generated by projectile impact was typically in the forms of well- or ill-developed ring or cone cracks with little presence of radial cracks.

An extensive experimental work for Pyroceram™ 9606 glass–ceramic was conducted to determine static fatigue at ambient temperature in distilled water. This work was an extension and companion of the previous work conducted in dynamic fatigue. Four different applied stresses ranging from 120 to 170 MPa was incorporated with a total of 20–23 test specimens used at each of four applied stresses. The slow crack growth (SCG) parameters n and D were found to be n = 19 and D = 45 with a coefficient of correlation of rcoef = 0.9653. The Weibull modulus of time to failure was in a range of msf = 1.6–1.9 with an average of msf = 1.7 ± 0.2. A life prediction using the previously determined dynamic fatigue data was in excellent agreement with the static fatigue data. The life prediction approach was also applied to advanced monolithic ceramics and ceramic matrix composites (CMCs) based on their dynamic and static fatigue data determined at elevated temperatures. All of these results indicated that a SCG mechanism governed by a power-law crack growth formulation was operative, a commonality of SCG in these materials systems.

Foreign object damage in gas turbines presents serious safety and financial concerns. As the aerospace industry draws closer to implementing ceramic matrix composites (CMCs) in gas turbines, the corresponding behavior in these materials after such events needs to be understood. To address this requirement, several silicon infiltrated fiber reinforced SiC/SiC coupons were impacted with high speed projectiles with velocities up to 360 m/s with an impact rig built at the University of Akron and NAVAIR. The resulting damage states were assessed using several non-destructive evaluation (NDE) techniques and compared to actual damage condition observed through the sectioning of impacted coupons. Ultimately, the true consequence of the damage was revealed by measuring the post-impact, residual strengths via uniaxial tensile tests to failure at both room and elevated temperatures. Lastly, the NDE results revealed a complicated damage morphology consisting of in-and-out-of plane damage that significantly affected the retained mechanical properties.Copyright © 2014 by ASME

The determination of “ultra” fast fracture strengths of five silicon nitride ceramics at elevated temperatures has been made by using constant stress-rate (“dynamic fatigue”) testing with a series of “ultra” fast test rates. The test materials included four monolithic and one SiC whisker-reinforced composite silicon nitrides. Of the five test materials, four silicon nitrides exhibited the elevated-temperature strengths that approached their respective room-temperature strengths at an “ultra” fast test rate of 3.3 × 104 MPa/s. This implies that slow crack growth responsible for elevated-temperature failure can be eliminated or minimized by using the “ultra” fast test rate. These ongoing experimental results have shed light on laying a theoretical and practical foundation on the concept and definition of elevated-temperature “inert” strength behavior of advanced ceramics.

An attempt was made to determine elevated-temperature, 'ultra'-fast fracture strengths of one alumina, two silicon nitrides and one silicon carbide by using constant stress-rate ('dynamic fatigue') testing with a series of 'ultra'-fast test rates. Of the materials tested, the alumina exhibited a convergence of strength at stress rates below 3.3 x 10(exp 4) MPa/s. The strength approached approximately the room-temperature inert strength. By contrast, the silicon nitrides and silicon carbide did not reveal a strength approach, but exhibited elevated-temperature strengths 10 and 20% lower than their respective room-temperature strengths. Although the analytical results imply that the elevated-temperature 'inert' strength of a ceramic material can be obtained by using sufficiently high stress rates, the experimental testing rates were only sufficient to demonstrate convergence for the alumina.

The solution of fatigue strength as a function of preloading in dynamic fatigue (constant stress-rate) testing was obtained analytically and numerically. The effect of preloading on dynamic fatigue strength decreases with increasing fatigue parameter (n), and for n ≥ 20 the effect is negligible up to a preloading of 90 percent. The solution was verified by dynamic fatigue experiments conducted with soda-lime glass and alumina specimens in room-temperature distilled water. This result showed that one can apply a preloading corresponding up to 90 percent of fatigue strength for most glass and ceramic materials, resulting in a dramatic saving of testing time in dynamic fatigue testing. The key feature that makes this technique feasible is that most of the slow crack growth under dynamic fatigue loading occurs close to failure time where the dynamic fatigue strength is defined.

The mode I, mode II, and combined mode I-mode II fracture behavior of ceria-doped tetragonal zirconia polycrystalline (Ce-TZP) ceramic was studied. The single-edge-precracked-beam (SEPB) samples were fractured using the asymmetric four-point-bend geometry. The ratio of mode I to mode II loading was varied by varying the degree of asymmetry in the four-point-bend geometry. The minimum strain energy density theory best described the mixed-mode fracture behavior of Ce-TZP with the mode I fracture toughness, K{sub IC} = 8.2 {+-} 0.6 MPa{center_dot}m{sup 1/2}, and the mode II fracture toughness, K{sub IIC} = 8.6 {+-} 1.3 MPa{center_dot}m{sup 1/2}.

Ceramic thermal barrier coatings (TBCs), attributed to their inherent brittle nature, are highly susceptible to damage by impacting foreign particles when the impacting kinetic energy exceeds certain limits. The damage is termed foreign object damage (FOD) in related turbine components and results in various issues/problems to coatings as well as to substrates from delamination to spallation to cracking to catastrophic failure depending on the severity of impact. The FOD testing was performed using a ballistic impact gun for turbine airfoil components coated with 7% yittria stabilized zirconia (7YSZ) by electron beam physical vapor deposit (EB-PVD). A range of impact velocities up to Mach 1 was applied with three different projectile materials of steel, silicon nitride, and glass balls. The damage was assessed and characterized in terms of impact velocity, projectile material, and remaining life of turbine components. An energy-balance approach was made to develop a model to predict delamination of the TBCs upon impact.

The fast fracture strength distribution of uniaxially ground, alpha silicon carbide was investigated as a function of grinding angle relative to the principal stress direction in flexure. Both as-ground and ground/annealed surfaces were investigated. The resulting flexural strength distributions were used to verify reliability models and predict the strength distribution of larger plate specimens tested in biaxial flexure. Complete fractography was done on the specimens. Failures occurred from agglomerates, machining cracks, or hybrid flaws that consisted of a machining crack located at a processing agglomerate. Annealing eliminated failures due to machining damage. Reliability analyses were performed using two and three parameter Weibull and Batdorf methodologies. The Weibull size effect was demonstrated for machining flaws. Mixed mode reliability models reasonably predicted the strength distributions of uniaxial flexure and biaxial plate specimens.Copyright © 1995 by ASME

The ratio of static to cyclic fatigue life, or 'h ratio', was obtained numerically for an indentation flaw system subjected to sinusoidal loading conditions. Emphasis was placed on developing a simple, quick lifetime prediction tool. The solution for the h ratio was compared with experimental static and cyclic fatigue data obtained from as-indented 96 wt.% alumina specimens tested in room-temperature distilled water.

Ceramic turbines have long promised to enable higher fuel efficiencies by accommodating higher temperatures without cooling, yet no engines with ceramic rotors are in production today. Studies cite life, reliability, and cost obstacles, often concluding that further improvements in the materials are required. In this paper, we assume instead that the problems could be circumvented by adjusting the engine design. Detailed analyses are conducted for two key life-limiting processes, water vapor erosion and slow crack growth, seeking engine design strategies for mitigating their effects. We show that highly recuperated engines generate extremely low levels of water vapor erosion, enabling lives exceeding 10,000 hs, without environmental barrier coatings. Recuperated engines are highly efficient at low pressure ratios, making low blade speeds practical. Many ceramic demonstration engines have had design point mean blade speeds near 550 m/s. A CARES/Life analysis of an example rotor designed for about half this value indicates vast improvements in slow crack growth-limited life and reliability. Halving the blade speed also reduces foreign object damage particle kinetic energy by a factor of four. In applications requiring very high fuel efficiency that can accept a recuperator, or in short-life simple cycle engines, ceramic turbines are ready for application today.

The combined effects of CMAS (calcium magnesium aluminosilicate) and FOD (foreign object damage) were determined in three different ceramic matrix composites (CMCs), two melt-infiltrated (MI) SiC/SiCs and one oxide/oxide. Foreign object damage was introduced at ambient temperature in CMC test targets using 1.6 mm steel ball projectiles at an impact velocity of 340 m/s utilizing a ballistic impact gun. One type of target support, partial support, was used. The impact-damaged test coupons were then CMAS-exposed at 1200°C for 10 hrs in air. Additional tests were also performed by exposing impact-damaged test coupons to a mixture of CMAS and salt (sodium sulfate) at 1200°C for 10 hrs in air. The combined effects of FOD and CMAS or CMAS/salt were quantified by determining the residual strengths of test coupons after exposures. Also, six different sands ingested into aeroengines were analyzed in their chemical compositions. The melt CMAS solid, melted at 1300°C and solidified, was assessed in its mechanical properties of microhardness and fracture toughness.

CMAS (Calcium-Magnesium-Aluminosilicate) has shown to induce some deleterious effects on yittria-stabilized-zirconia (YSZ) based thermal barrier coatings (TBCs) of hot section components of aeroengines. The effects were shown to be dependent on the types and operating conditions of engines/components. The work presented here explored how CMAS would affect ceramic matrix composites (CMCs) in terms of strength degradation. Four different, gas-turbine grade CMCs were utilized including two types of MI SiC/SiCs and other two types of oxides/oxides (N720/aluminisilicate and N720/alumina). Test specimens in a simple flexure configuration were CMAS-treated at 1200 °C in air under either isothermal or thermal cycling condition. The effects of CMAS were quantified via residual strengths of treated test specimens. Strength degradation with respect to as-received strengths ranged from 10 to 20% depending on the types of CMCs. It was further observed that significant degradation of strength up to 90% occurred in an oxide/oxide CMC when sodium sulfate was added to CMAS.

Foreign object damage (FOD) phenomena of two gas-turbine grade silicon nitrides (AS800 and SN282) were assessed at ambient temperature applying impact velocities from 20 to 300 m/s using 1.59-mm diameter hardened steel ball projectiles. Targets in a flexural configuration with two different sizes (thicknesses) of 1 and 2 mm were ballistic-impacted under a fully supported condition. The severity of impact damage, as well as the degree of post-impact strength degradation, increased with increasing impact velocity, increased with decreasing target size, and was greater in SN282 than in AS800 silicon nitride. The critical impact velocity where targets fractured catastrophically decreased with decreasing target size and was lower in SN282 than in AS800. Overall, FOD by steel projectiles was significantly less than that by silicon-nitride ceramic counterparts, due to much decreased Hertzian contact stresses. A correlation of backside cracking velocity versus target size was made based on a simplified elastic foundation analysis.Copyright © 2011 by ASME

The crack-healing behavior of identation cracks and large precracks in 96 wt.% alumina was studied as a function of annealing temperature in air and inert environments. Crack healing occurred at annealing temperatures of 800 °C and above in both air and inert (argon) environments, indicating its cause as transport of existing material to the crack plane, and resulted in increased strength and fracture toughness. However, the resulting phase assemblage in the crack plane was susceptible to fatigue at room temperature in distilled water, as evidenced by a somewhat low fatigue parameter of N = 68. Furthermore, polished-and-annealed samples were more susceptible to fatigue at room temperature in air than as-polished samples.

An effort is made to deepen understanding of environmental effects on the fracture toughness of an alumina composition that contains a residual glassy phase, by ascertaining the fracture toughness under atmospheric conditions in such varied environments as air distilled water, silicone oil, and liquid nitrogen. Fracture toughness was determined via the single-edge-precracked beam technique. Weibull strength parameters are compared for polished specimens tested both in air and silicone environments.

The mechanism of formation of the perovskite phase and the dielectric properties of Pb(Zn,Mg)1/3Nb2/3O3 (PZMN) ceramics were examined using two different types of columbite precursors, (Mg,Zn)Nb2O6 (MZN) and MgNb2O6+ ZnNb2O6 (MN + ZN). The formation of perovskite phase in the PbO + MN + ZN system is characterized by an initial rapid formation of Mg-rich perovskite phase, followed by a sluggish formation of Zn-rich perovskite phase. On the other hand, due to the formation of pyrochlore phase of mixed divalent cations Pb2–x(Zn,Mg)yNb2−yO7−x−3y/2, the pyrochlore/perovskite transformation in the PbO + MZN system proceeded uniformly with a spatial homogeneity. Further analysis suggested that the formation of perovskite phase is a diffusion-controlled process. The degree of diffuseness of the rhombohedral/cubic phase transition (DPT) is higher in the PbO + MN + ZN system than in the PbO + MZN specimen for T > Tmax (temperature of the dielectric permittivity maximum), indicating a broadened compositional distribution of the B-site cations in the PbO + MN + ZN system.

An 8-wt pct-scandia silicon nitride with an elongated grain structure was fabricated. The material exhibited high fracture toughness and a rising R-curve as measured by the indentation strength technique. The 'toughening' exponent m was found to be m about 0.1. The high fracture toughness and R-curve behavior was attributed mainly to bridging of the crack faces by the elongated grains. The crack closure (bridging) stress distribution in the wake region of the crack tip was estimated as a function of crack size from the R-curve data, with an arbitrarily assumed distribution function.

It is shown that limited oxidation of an MgO-containing, hot-pressed silicon nitride ceramic at 800 deg C and above results in increased strength due to crack healing. Slight oxidation of the surface produces enstatite and cristobalite which fills in cracks. More extensive oxidation leads to strength degradation due to the formation of new flaws by the evolution of N2 gas at the surface. The apparent fracture toughness also increased at 800 deg C and above due to oxidation. Bonds formed between the two surfaces of the crack during oxidation leads to a reduction in stress intensity at the crack tip, suggesting that valid high-temperature toughness values cannot be obtained in an air environment. The increase in strength due to crack healing by oxidation can be achieved without compromising the fatigue properties of the silicon nitride ceramic.

Preface ix FIBER COMPOSITES. Influence of Fiber Orientation on the Mechanical Properties and Microstructure of C/C-SiC Composite Plates Produced by Wet Filament Winding Technique 3 Fabian Breede, Severin Hof mann, Enrico Klatt, and Sandrine Denis High-Temperature Interlaminar Tension Test Method Development for Ceramic Matrix Composites 11 Todd Z. Engel, Wayne S. Steffier, and Tony Magaldi Life Limiting Behavior of Ceramic Matrix Composites Under Interlaminar Shear at Elevated Temperatures 23 Sung R. Choi NANO-CERAMICS AND COMPOSITES. Effect of Coating Parameters on the Electrodeposition of Nickel Containing Nano-Sized Alumina Particles 41 R. K. Saha, S. Mohamed, and T. I. Khan Synthesis of AI203-TiC Nano-Composite Particles by a Novel Electro-Plasma Process 51 Kaiyang Wang, Peigen Zhang, Jiandong Liang, and S.M. Guo TESTING, CHARACTERIZATION, AND MICROSTRUCTUREPROPERTY RELATIONSHIPS. Research on Tribological Properties of Reactive Sintered Si3N4-Based Composite Ceramic 63 Hai-long Ma, Chong Cui, Xing Li, Yuan-ting Wang, and Hai-jun Zhou Preparation of Nano and Micron Sized ZrO2 Dispersed Al2O3 Ceramic Composites and Study Their Hardness and Fracture Toughnesses 75 Kuntal Maiti and Anjan Sil Influence of Particle Size Distribution of Wollastonite on the Mechanical Properties of CBPCs (Chemically Bonded Phosphate Ceramics) 85 H. A. Colorado, C. Hiel, and H. T. Hahn Foreign Object Damage in an N720/Alumina Oxide/Oxide Ceramic Matrix Composite under Tensile Loading 99 D. Calvin Faucett and Sung R. Choi FUNCTIONALLY GRADED MATERIALS. Thermally Sprayed Functionally Graded Materials 109 Pavel Chraska and Tomae Chraska CERAMIC PROCESSING. Development of Ultra-High Temperature Stable Ceramics by Reactive Infiltration Processes* 123 R. Voigt, W. Krenkel, G. Motz, and A. Can Shape Evolution of Spanning Structures Fabricated by Direct-Write Assembly of Concentrated Colloidal Gels 131 Cheng Zhu and James E. Smay COMPOSITES PROCESSING. Colloidal Processing of Ceramic-Ceramic and Ceramic-Metal Composites 147 Rodrigo Moreno Processing of Nitride Porous Ceramic Composites via Hybrid Precursor System Chemical Vapor Deposition (HYSYCVD)/Direct Nitridation (DN) 161 M. I. Pech-Canul and J. C. Flores-Garcia Effect of Nano-SiO2 on Microstructure, Interface and Mechanical Properties of Whisker-Reinforced Cement Composites 173 Mingli Cao and Jianqiang Wei DIRECTIONAL SOLIDIFICATION AND MICROWAVE PROCESSING Engineered Self-Organized Microstructures Using Directional Solidification of Eutectics 185 V. M. Orera, J. I Pena, A. Larrea, R. I. Merino, and P. B. Oliete Effect of Different Fuels on the Microwave-Assisted Combustion Synthesis of Ni0.5Zn0.5Fe1.95Sm0.05O4 Ferrites 197 A. C. F. M. Costa, D. A. Vieira, V. C. Diniz, H. L. Lira, D. R. Cornejo, and R. H. G. A. Kiminami Author Index 207 *Paper Presented at the 8th Pacific Rim Conference on Ceramic and Glass Technology, May 31-June 5, 2009

Strength of three continuous fiber-reinforced ceramic composites, including SiC/CAS-11, SiC/MAS-5 and SiC/SiC, was determined as a function of test rate in air at 1100 - 1200 C. All three composite materials exhibited a strong dependency of strength on test rate, similar to the behavior observed in many advanced monolithic ceramics at elevated temperatures. The application of the preloading technique as well as the prediction of life from one loading configuration (constant stress-rate) to another (constant stress loading) suggested that the overall macroscopic failure mechanism of the composites would be the one governed by a power-law tyw of damage evolution/accumulation, analogous to slow crack growth commonly observed in advanced monolithic ceramics. It was further found that constant stress-rate testing could be used as an alternative to life prediction test methodology even for the composite materials at least for the short range of lifetime.

The error associated with strength measurements made with fixed-roller and V-grooved roller figures was studied with a variety of roller and figure material combinations. Both the fixed-roller and the V-grooved fixtures yielded an error in stress of about 4 to 7%. They also exhibited an appreciable amount of load strain hysteresis. Dynamic fatigue testing of 96 wt.% alumina verified the error: the fatigue strength was overestimated by 7 to 9% for the fixed roller configuration. The source of error is exclusively attributed to friction between the rollers and the specimen surface, as verified by previous investigators.

Thermal fatigue and fracture behavior of plasma-sprayed ceramic thermal barrier coatings has been investigated under high heat flux and thermal cyclic conditions. The coating crack propagation is studied under laser heat flux cyclic thermal loading, and is correlated with dynamic fatigue and strength test results. The coating stress response and inelasticity, fatigue and creep interactions, and interface damage mechanisms during dynamic thermal fatigue processes are emphasized.

A barium calcium aluminosilicate (BCAS) glass of composition (mol%) 35BaO-15CaO-5Al 2 O 3 -108 2 O 3 -35SiO 2 that is being investigated as sealing material for planar solid oxide fuel cells was reinforced with alumina platelets or 3 mol% yttria-stabilized zirconia (YSZ) particulates to improve its strength and fracture toughness. For the same ceramic content, composites containing alumina platelets showed much higher strength and fracture toughness than those with YSZ. For 30 mol% reinforcement, flexure strength of the glass improved by 230 % and 130 %, respectively, for alumina and YSZ reinforced composites, while fracture toughness improved by 350 and 120%. The 20 and 30 mol% alumina composites exhibited rising R-curve behavior. The increase in elastic modulus was more predominant for alumina containing composites than for YSZ composites. Addition of alumina did not have much effect on the density but addition of YSZ increased the density linearly. Effect of reinforcement on hardness was greater for alumina containing than for YSZ containing composites. The YSZ reinforced glass composites revealed more enhanced viscous behavior than the alumina counterparts at both 800 and 1000 °C.

Previously derived solutions of fatigue strength as a function of preloading were verified by applying preloads to elevated temperature dynamic fatigue tests of 96 wt% alumina at 1000 C and NC 132 silicon nitride at 1100 C. The technique was found very useful in identification and control of the governing failure mechanism when multiple failure mechanisms, such as slow crack growth, creep and oxidation occurred simultaneously at elevated temperatures.

An attempt to determine high temperature "inert" strengths of several silicon nitride ceramics was made by using dynamic fatigue testing at the very high stressing rate of 2 x 106 MPa/min. The fatigue strengths of all the silicon nitride materials exhibited a well-defined convergence to the corresponding room-temperature inert strengths as stressing rate was increased. The result showed that the high temperature Inert" strength of a ceramic material can be obtained by using sufficiently high stressing rates (greater or = 2 x 10(exp 6) MPa/min) at which fatigue strength converges to a specific value. The specific value can be close to the room-temperature strength of the material.

Foreign object damage (FOD) behavior of an oxide/oxide (N720/AS) ceramic matrix composite (CMC) was determined at ambient temperature using impact velocities ranging from 100 to 400 m/s by 1.59-mm diameter steel-ball projectiles. Two different support configurations of target specimens were used: fully supported and partially supported. The degree of post-impact strength degradation increased with increasing impact velocity, and was greater in a partially supported configuration than in a fully supported one. For the fully supported configuration, frontal contact stress played a major role in generating composite damage, while for the partially supported case both frontal contact and backside flexure stresses were the combined sources of damage generation. The oxide/oxide composite was able to survive high energy (∼1.3 J) impacts without complete structural failure. The degree of relative post-impact strength degradation of the oxide/oxide composite was similar to that of an advanced SiC/SiC composite observed from a previous study, regardless of the type of specimen support. Like the SiC/SiC composite, impact-damage tolerance was greater in the oxide/oxide than in monolithic silicon nitride ceramics for impact velocities >300 m/s.

Interlaminar crack growth resistances were determined for five different SiC fiber-reinforced ceramic matrix composites (CMCs) including three gas-turbine grade melt-infiltrated SiC/SiC composites. Modes I and II crack growth resistances, GI and GII, were evaluated at ambient temperature using double cantilever beam and end notched flexure methods, respectively. The CMCs exhibited GI =200-500 J/m2 and GII=200-900 J/m2. Most of the CMCs, except the SiC/CAS composite, showed rising R-curve behavior either in Mode I or in Mode II, presumably attributed to fiber bridging (in Modes I and II) and frictional constraint (in Mode II) in the wake region of a propagating crack. A glass fiber-reinforced epoxy polymer matrix composite, used as comparison, showed R-curve behavior and typically 2-3 and 8 times greater in GI and GII, respectively, than the CMCs. Experimental error analysis concerning the effect of the off-the-center of a crack plane on GI and GII was also made.

Interlaminar crack growth resistances were evaluated for five different SiC fiber-reinforced ceramic matrix composites (CMCs) including three gas-turbine grade MI SiC/SiC composites. Modes I and II crack growth resistances, GI and GII, were determined at ambient temperature using double cantilever beam (DCB) and end notched flexure (ENF) methods, respectively. The CMCs exhibited GI = 200–500 J/m2 and GII = 200–900 J/m2. All the composites (except for one SiC/CAS composite) showed rising R-curve behavior either in mode I or in mode II, presumably attributed to fiber bridging (in modes I and II) and frictional constraint (mode II) in the wake region of a propagating crack. A glass fiber-reinforced epoxy polymer matrix composite, used as comparison, showed typically 2-3 and 8 times greater in GI and GII, respectively, compared to the CMCs. Experimental error analysis regarding the effect of the off-the-center of a crack plane on GI and GII was also made.

Foreign object damage (FOD) behavior of AS800 silicon nitride was determined using four different projectile materials at ambient temperature. The target test specimens rigidly supported were impacted at their centers by spherical projectiles with a diameter of 1.59 mm. Four different types of projectiles were used including hardened steel balls, annealed steel balls, silicon nitride balls, and brass balls. Post-impact strength of each target specimen impacted was determined as a function of impact velocity to better understand the severity of local impact damage. The critical impact velocity where target specimens fail upon impact was highest with brass balls, lowest with ceramic ball, and intermediate with annealed and hardened steel balls. Degree of strength degradation upon impact followed the same order as in the critical impact velocity with respect to projectile materials. For steel balls, hardened projectiles yielded more significant impact damage than annealed counterparts. The most important material parameter affecting FOD was identified as hardness of projectiles and was correlated in terms of critical impact velocity, impact deformation, and impact load.Copyright © 2005 by ASME

The determination of “ultra”-fast fracture strengths of a total of 17 advanced ceramics at elevated temperatures has been made by using constant stress-rate testing in flexure with a series of “ultra”-fast test rates. The test materials included two aluminas, eleven monolithic silicon nitrides, two SiC whisker-reinforced composite silicon nitrides, and two silicon carbides. Of the 17 ceramic materials tested, 15 advanced ceramics exhibited elevated-temperature strengths that approached (within 90%) their respective room-temperature strengths at an “ultra”-fast test rate of 3.3 × 104 MPa/s. This indicates that slow crack growth responsible for elevated-temperature failure can be eliminated or minimized by using a sufficiently fast test rate. These ongoing experimental results have shed light on laying a theoretical and practical foundation on the concept and definition of elevated-temperature “inert” strength behavior of advanced ceramics. The elevated-temperature, “inert” strength of a ceramic material could be determined by using test rates equal to or greater than the “ultra”-fast test rate of 3.3 × 104 MPa/s

Preloading technique was used as a means of an accelerated testing methodology in constant stress-rate (“dynamic fatigue”) testing for two different brittle materials. The theory developed previously for fatigue strength as a function of preload was further verified through extensive constant stress-rate testing for glass-ceramic and CRT glass in room-temperature distilled water. The preloading technique was also used in this study to identify the prevailing failure mechanisms at elevated temperatures, particularly at lower test rates in which a series of mechanisms would be associated simultaneously with material failure, resulting in significant strength increase or decrease. Two different advanced ceramics including SiC whisker-reinforced composite silicon nitride and 96 wt% alumina were used at elevated temperatures. It was found that the preloading technique can be used as an additional tool to pinpoint the dominant failure mechanism that is associated with such a phenomenon of considerable strength increase or decrease.Copyright © 2001 by ASME

The configuration and the strength response of indentation flaws in silicon nitrides are described as a function of indentation loads primarily near the threshold level. Test materials under consideration include 30 vol percent SiC whisker-reinforced composite silicon nitride and similar monolithic silicon nitride. The results of strength testing show that, at indentation loads less than or equal to 1.96, a number of the specimens fail from the intrinsic flaws of the materials rather than from the indent sites, causing a distinct trend to reach a plateau, at a level corresponding to the as-received strength of the material. The ratio of the mirror constant to the fracture toughness for the composite and monolithic materials is found to be 1.44 and 1.51, respectively.